Especially in Europe, there are local-area wireless communication systems including a plurality of base stations and a main device in conformity to DECT (digital enhanced cordless telecommunications) standard. The DECT standard employs TDMA-TDD (time division multiple access-time division duplex), and as one configuration thereof, a configuration that substantially periodically conducts calls and communications with one cycle having 12 slots in total of 6 transmission slots and 6 reception slots. When a communication system of the DECT standard is used for telephone calls, a high sound quality can be expected, and a reduction in the costs of the base stations and the terminal can be also expected.
On the other hand, wireless communication systems with an access point that communicates through a wireless LAN (hereinafter referred WiFi) using a communication standard of IEEE 802.11 series is also one of widespread communication systems, which are unfit for the telephone communication but suitable for a fast communication intended for personal computers. Also, in the WiFi (802.11), if the terminal is always operated in a reception mode, a communication from the base stations reaches the terminal substantially in real time. However, if the terminal is operated in a sleep mode, the base station repeats temporal activation at timing of beacon periodically transmitted by the base station. When TIM (traffic indication map) information is included in the beacon transmitted by the base station, the terminal that has temporarily been activated finds whether the base station holds a data frame addressed to the subject base station, or not. Also, when DTIM information is included in a beacon transmitted by the base station, the terminal finds that the base station holds a data frame for broadcast or multicast.
The base station that conducts a WiFi communication broadcasts the beacon including TIM at respective beacon intervals. For example, a certain terminal (terminal A) is activated for receiving the TIM at every two intervals. On the other hand, the other terminal (terminal B) is activated for receiving the TIM at every three intervals. Accordingly, in the case of the second beacon, only the terminal A is activated for receiving the TIM.
When data frames for allowing the base station to broadcast the data frames addressed to the respective terminals are buffered, not the TIM but DTIM is included in the beacon transmitted by the base station, which is indicative of the presence of data addressed to the terminal. For example, the terminal A that has received the second beacon and the DTIM finds that data present in the base station is data for broadcast. Therefore, when the terminal A finds that the frame is buffered by the base station, the terminal A issues PS-Poll to the base station, and the data frame is transmitted from the base station to the terminal A in response to the PS-Poll. After completion of this communication, the terminal A returns to the sleep mode. During the above operation, the terminal B is kept in the sleep mode. In the case of the third beacon, the terminal A remains slept, the terminal B is activated for receiving the beacon, and the terminal B receives the beacon transmitted by the base station. If the DTIM information is included in the beacon received by the terminal B, the terminal B issues PS-Poll to the base station, and the data frame is transmitted from the base station to the terminal B. In the case of the fourth beacon, the terminal A is activated for receiving the beacon.